Radar-compatible data link system (U)

ABSTRACT

The invention provides a radar system capable of transmitting data communications concurrently with and on a non-interfering basis with the normal radar functions. A continuous-wave carrier frequency is generated and transmitted to perform the normal radar function such as the processing of doppler data. For data communication purposes, the carrier signal is pulse modulated at selected pulse repetition frequencies with the selected PRF transmitted as first-order sideband components of the fixed carrier frequency. PRF selection is made in accordance with input digital data. A remotely-disposed command link receiver processes the incoming carrier wave and its sideband components to reconstruct the information contained in the transmitted data rates. Sensitivity and optimum detection by the command link receiver is achieved in a special manner that includes a phase locking of the received carrier frequency to the generated carrier frequency.

BACKGROUND OF THE INVENTION

The present invention relates to radar systems and, in particular, tosystems having a communication capability added to its designed radarcapability.

Systems of the type under consideration are used for a number ofpurposes one of which is to track semi-active radar guided missiles orthe like and, as needed, to transmit course-correction commands or thelike over a data link. By way of illustration, such a system may utilizepulse doppler radar and, to achieve the desired data communicationcapability, some type of carrier frequency modulation is employed.However, since the carrier frequency itself is modulated the radarfunction must be interrupted during each message transmission. In otherwords, the radar transmitter must be time-shared between the radar andthe data link functions. Similar situations exist in comparable systemssuch as those used for secure communication purposes or for IFFinterogatoins. For example, secure communication radars may employ ahighly directional transmitter antenna for message purposes but, againthe transmitter must be time-shared rather than being continuouslyavailable for uninterrupted operation in either mode.

According to the present invention, a radar compatible data link systemis provided by utilizing pulse repetition modulation (PRF) for the datalink function. More specifically, the transmitter section of the systemgenerates and transmits a radar carrier wave the transmitted frequencyof which remains intact or unmodulated to permit the radar function tocontinue unaffected by the data transmission. As indicated, PRFmodulation provides the data link capability and, for this purpose, thesystem utilizes a plurality of PRF generators of differing repetitionrates. The output of theses generators are selectively applied to pulsemodulate the carrier wave. A digital input, in turn, controls theselection of the PRF generators to the extent that, for example, abinary ‘1’ will select one of the generators, while a binary ‘0’ willselect another. The transmitted wave thus has the unmodulated carrierfrequency to provide the radar function. It also contains the messagedata in the PRF sideband components of the wave. A command receiverselectively processes the sideband information to reconstruct thedigital data.

DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the accompanying drawings of which;

FIG. 1 is a schematic portrayal of one manner in which the present radarcompatible link system is to be employed;

FIG. 2 is a block diagram of the radar transmitter section of thesystem, and

FIG. 3 is a block diagram of the command receiver section of the system.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the major components of the system include a radarsignal transmitter 1, a radar return-signal receiver 2 and a commandreceiver 3 which, for illustrative purposes, is carried by a semi-activeguided missile 4 the flight of which is being tracked and controlled bythe radar. An antenna 6 transmits the radar signal and receives thereturn although, of course, separate antennas can be used for these twopurposes. If security of the commands or communications received bycommand receiver 3 is a critical factor antenna 6 can be made highlydirectional by utilizing a dish antenna having about an appropriate beamwidth, such as a 1.5-20 width. The secure communication aspects of thepresent system are considered to be an important application of theinventive concept although, as indicated, the primary concept is toassure uninterrupted or, in other words, simultaneous functioning ofboth the data transmitting and the radar return processing of thesystem.

As will be appreciated, the present system is particularly suited for adoppler radar which is continuously receiving range data from arapidly-moving vehicle and which also is adapted to transmit messages.According to well-known principles, the vehicle movement producesvariations of the transmitted carrier wave and these variations arecontinuously integrated by the receiver to produce the desired radardata. Since the data is a function of the variations in the frequency ofthe transmitted carrier wave, it at least is highly preferably that thefrequency of the carrier wave remain at a fixed value. As has beenexplained, the need for maintaining the carrier wave at a fixedfrequency has seriously complicated the used of current pulsed dopplerradars for communication purposes. The problem is that these currentradars achieve their data communication capability by modulating or, inother words, actually varying the frequency of the carrier wave ratherthan permitting it to remain fixed. The result is that the doppler dataprocessing must be interrupted during message transmission and,consequently, the transmitter of these current radars must be timeshared. The present system avoids the difficulty by utilizing pulserepetition frequency (PRF) modulation to achieve the data communicationcapability and, since the carrier wave used for doppler or other radarpurposes remains fixed or unmodulated, the radar functions can beperformed on an uninterrupted basis.

The transmitter portion of the present system is illustrated in blockform in FIG. 2. As will be noted each of the blocks of this figure, aswell as FIG. 3, are functionally identified in the drawing. It also isto be noted that the various components used in the system may beprovided in conventional manners so that actual implementation of thesystem should present no problem. In other words, all of the componentsof the present system are well-known, commercially-available items whichshould require no detailed identification or description. The operatingcharacteristics of the system, of course, can be widely varied bycomponent selection.

Referring to FIG. 2, it will be noted that the carrier wave to betransmitted by antenna 6 is derived in the usual manner from a crystaloscillator 7 the output frequency of which is multiplied by N in amultiplier 8 for application to a modulator 9 and a power amplifier 11.

A particular feature of the system resides in the manner in which thecarrier wave is modulated. As explained, it is essential to the presentinvention that the frequency of the carrier wave remain at a fixed levelso as to permit continuous use of the radar functions of the system. Toachieve this purpose, the carrier signal is pulse modulated at aselected pulse repetition frequency (PRF) before being applied to poweramplifier 11. As shown in FIG. 2, two PRF generators 12 and 13 areemployed to pulse modulate the carrier. Functionally considered, one orthe other of these generators selectively modulates the carrier and thegenerator selection is controlled by appropriate switches 14 and 16which, in turn, are controlled by a switch driver 17 the operation ofwhich is responsive to the binary state of digital data applied to thedriver from a digital data source 18. A summer 19 is used at the outputsof switches 14 and 16, although as will be appreciated, the summer inthe illustrated system simply is summing a ‘1’ and a ‘0’ format beforeapplication of the selected PRF to modulator 9. In other words, in theillustrated form, PRF selection is made in accordance with the inputdigital data to the extent that a logic “one” selects one PRF and alogic “zero” selects the other PRF with the transmitter being modulatedat the selected PRF for the duration of the data link pulse.Consequently, the waveform transmitted by antenna 6 constitutes a seriesof pulses of the fixed carrier wave frequency derived from crystaloscillator 7 and the information or message data to be transmitted iscontained in sideband components of the carrier wave. The net result isa transmission of a fixed carrier wave frequency at a repetition ratewhich is dependent upon the selected PRF generator so that theinformation contained in the transmitted signal resides in thetransmitted data rate rather than an actual variation of the carrierwave frequency.

A command receiver, such as is shown in FIG. 3, is employed to derivedigital data information by detecting the data rate of the incomingsignal and reconstructing the applied digital data from variations inthe pulse rate. This receiver, as stated, is carried by the missile orother object that is to receive the message data. Concurrently, thedoppler or other radar return information is continuously anduninterruptedly obtained by receiver 2 of FIG. 2. Receiver 2, as hasbeen indicated, may be considered as a doppler data receiver adapted todetect and continuously integrate variations in the fixed carrier wavefrequency of the transmitter. Implementation of this receiver can beachieved in any of the well known manners, and, of course, it isanticipated that other types of radar return processors may be employed.It further should be appreciated that the system is shown in itssimplest form in which only a pair of PRF generators is employed totransmit one bit of information per data link pulses. Clearly, however,the invention is equally applicable to more complex coding schemes whichuse more than two PRF's to transmit more than one bit of information perdata link pulse.

Command receiver 3 is provided with a conventional antenna 20 to receivethe signal transmitted by the FIG. 2 radar transmitter. This receivergenerally is a superhetrodyne receiver using a linear AGC'd IFamplifier. More specifically, the incoming signal is mixed in a mixer 21with the output of a local oscillator 22 and applied to an IF amplifier23 controlled by an AGC component 24. The output of IF amplifier 23 thenis applied to a bandpass filter 26 which is selected to pass both thecarrier wave frequency and both first order sidebands produced by thepulse modulation. Preferably, the output of this filter is an AM signalwith a modulation index greater than 1.0. Although not illustrated inFIG. 3, this requirement makes it possible to recover the PRF modulationafter processing the received signal through a limiter IF amplifierinstead of a linear IF amplifier. Although the use of the linear IFamplifier may produce greater sensitivity, the use of a limiter IFamplifier is somewhat less complex. Thus, as illustrated, IF amplifier27 employed at the output of bandpass filter 26 can be of either typeand, as shown, this amplifier also is AGC'd.

The AGC'd IF amplifier output is supplied to a mixer 28 which is used toprovide coherent AM detection. As probably will be appreciated, thereceiver processing as a whole can be considered as a coherent receiver.To achieve optimum detection, the receiver also employs a phase-lockloop which phase-locks the carrier in such a manner that coherentaddition of the sideband signals can be achieved in mixer 28. Such aphase-lock loop also permits noise to be processed linearly and added inan rms manner. As shown, the phase-lock loop includes a bandpass filter29 which is selected to pass only the center or carrier signal so thatonly this carrier signal is applied to the loop. A limiter 31 preferablyis introduced to prevent frequency centroiding in cases where multipathinterference produces a second carrier signal differing in frequencywith the primary or direct signal. The limiter permits the strongersignal to be passed on so as to provide it as the carrier frequency towhich the sideband signals are added in mixer 28. The functional part ofthe loop consists primarily of a phase detector 32, a loop filter 33 anda voltage control oscillator (VCO)34 which, in the customary manner,provides a signal phase locked to the carrier. As will be appreciated,the use of the phase-lock loop is important since it significantlyimproves subsequent detection and processing of the sideband componentswhich contain the data or message being transmitted. However, it isrecognized that phase locking of the type achieved by the loop is wellknown in the art and its end results can be achieved in a number ofdifferent manners. In general the receiver system is one in which AMdetection is achieved by forming the product of the AM signal and asinusoid that is phase locked to the carrier so as to result in optimumdetection which, as explained, is achieved principally by the coherentaddition of the sideband signals. The detected output, which is theoutput of mixer 28, then is supplied to a pair of bandpass filters 36and 37. As will be noted, these filters also are identified in FIG. 3 asbandpass filter (PRF No. 1) and bandpass filter (PRF No. 2). Thedesignations PRF No. 1 and PRF No. 2 relate back to the transmittersection of the system which includes the pair of PRF generators alsoidentified as PRF No. 1 and PRF No. 2. In other words, as would beexpected, the centerlines of filters 36 and 37 are matched to the pulserates of generators 12 and 13 so as to recover and permit furtherprocessing of the data contained in the sidebands produced by modulatingthe carrier with one or the other of the PRF generator outputs. Afterfiltering, the filter outputs are detected in the usual manner byenvelope detectors 38 and 39 and the output of these detectors thensupplied to a high gain differential amplifier 41 for conversion to adigital data format.

In general, the described arrangement provides a relatively simplemanner of providing data communication in a system that uses a pulseddoppler radar with the data link of the system being on anon-interfering basis with the radar function. In addition, it can beimplemented to provide improved signal-to-noise ratio so as to permit areduction in bit error rate and an increased data link sensitivity.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

1. A radar-compatible data link system comprising the combination of:radar signal transmitting means; radar signal return processing means,and a remotely-disposed radar signal receiving means: said transmittingmeans including: means for generating a fixed continuous wave carrierfrequency, means for generating a plurality of differing pulserepetition frequencies (PRF'S) means for selectively modulating saidfixed carrier frequency with said generated PRF's for producingtransmitter output signal pulses having said fixed carrier frequency asa center frequency component and a sideband component formed by saidselected PRF, a digital data source for introducing into the system saiddata link information, and means controlled by said digital informationfor repetitively selecting the modulating PRF; said return signalprocessing means including: means for processing said fixed carrierfrequency component to produce radar-return information, and saidremotely-disposed receiving means including: a matched band-pass filterfor each of said PRF's, said filters each being adapted to pass as anoutput sideband components produced by a particular PRF generator, anddifferential amplifier means receivably coupled to said filter outputfor processing said filter means output for reconstructing said datalink information whereby said system is capable of processing radarreturn data concurrently with the transmission of said digital datainformation.
 2. The system of claim 1 wherein said transmitting meansincludes: a directional transmitter antenna having a narrow beamwidthadapted for secure communications purposes.
 3. The system of claim 1wherein said control means for selecting the modulating PRF includes:switching means coupled between an output of said PRF generators andsaid modulating means, and switch driver means operatively coupled tosaid data source and to said switching means for controlling saidswitches responsively to said digital data whereby the selection of thePRF is responsive to said digital data.
 4. The system of claim 1 whereinsaid remotely-disposed receiving means further includes: means forheterodyning the incoming signal to produce an intermediate frequencyoutput, a selective bandpass filter coupled to said heterodyned outputfor removing said sideband components and passing as an output saidfixed carrier frequency component, and a mixer for receiving and mixingsaid heterodyned output with said selective bandpass filter output, saidmixer having an output coupled to said matched pass-band filters.
 5. Thesystem of claim 4 further including: phase-lock circuit means coupledbetween the output of said selective bandpass filter and said mixer,said circuit-receiving the output of said filter and locking said outputto the phase of said generated carrier frequency.
 6. The system of claim5 where said phase-lock loop includes: a variable controlled oscillatorfor supplying a signal phase locked to said generated carrier frequencyfor enabling coherent addition of said sideband components in saidmixer.